Electron transfer reactions are crucial steps in a variety of biological transformations ranging from photosynthesis to aerobic respiration. Studies of electron transfer reactions in both chemical and biological systems have led to the development of a large body of knowledge and a strong theoretical base, which describes the rate of electron transfer in terms of a definable set of parameters.
Electronic tunneling in proteins and other biological molecules occurs in reactions where the electronic interaction of the redox centers is relatively weak. Semiclassical theory reaction predicts that the reaction rate for electron transfer depends on the driving force (-.DELTA.G.degree.), a nuclear reorganization parameter (.lambda.), and the electronic-coupling strength (H.sub.AB) between the reactants and products at the transition state, according to the following equation: EQU k.sub.ET =(4.pi..sup.3 /h.sup.2 .lambda.k.sub.B T).sup.1/2 (H.sub.AB).sup.2 exp[(-.DELTA.G.degree.+.lambda.).sup.2 /.lambda.k.sub.B T]
The nuclear reorginzation energy, .lambda., in the equation above is defined as the energy of the reactants at the equilibrium nuclear configuration of the products. There are two components of .lambda.; "outer sphere" effects (.lambda..sub.o) and "inner sphere" effects (.lambda..sub.i). For electron transfer reactions in polar solvents, the dominant contribution to .lambda. arises from the reorientation of solvent molecules in response to the change in charge distribution of the reactants. The second component of .lambda. comes from the changes in bond lengths and angles due to changes in the oxidation state of the donors and acceptors.
It is an object of the present invention to provide methods for the detection of target analytes exploiting changes in the solvent reorganization energy of electron transfer reactions.